Radio systems, e.g. in the mobile radio sector, often use a common antenna for transmit and receive signals. These transmit and receive signals use different frequency ranges, and the antenna must be suitable for transmitting and receiving in both frequency ranges. A suitable frequency filtering element is required to separate the transmit and receive signals, which element is used to forward transmit signals from the transmitter to the antenna and receive signals from the antenna to the receiver. Among other devices, high-frequency filters having a coaxial structure are used today to separate the transmit and receive signals.
For example, a pair of high-frequency filters can be used which both allow a specific frequency band to pass (band pass filters). Alternatively, a pair of high-frequency filters can be used which both block a specific frequency band (band stop filters). Furthermore, a pair of high-frequency filters can be used in which one filter lets frequencies under a frequency between the transmit and receive band pass and blocks frequencies above that frequency (low pass filter) and the other filter blocks frequencies below a frequency between the transmit and receive band and lets frequencies above it pass (high pass filter). Other combinations of the filter types just mentioned are conceivable. High-frequency filters are often produced in the form of coaxial TEM resonators. These resonators can be manufactured economically and at low cost from milled or cast parts and ensure high electrical quality and a relatively high temperature stability.
A single coaxial resonator produced using milling or casting techniques consists, for example, of a cylindrical inner conductor and a cylindrical outer conductor. It is likewise possible that the inner conductor and/or the outer conductor has a regular n-polygonal cross section in the transverse direction to the inner conductor. The inner and outer conductors are interconnected at one end across a large area by an electrically conductive layer (typically shorted by an electrically conductive bottom). Typically, air is used as a dielectric between the inner and outer conductors.
The mechanical length of such a resonator (with air as dielectric) corresponds to one fourth of its electric wavelength. The resonance frequency of the coaxial resonator is determined by its mechanical length. The longer the inner conductor, the greater the wavelength and the lower the resonance frequency. Electric coupling between the two resonators is the weaker the farther the inner conductors of two resonators are away from one another and the smaller the coupling aperture between the inner conductors.
A large number of proposals have been made to improve such resonators.
For example, EP 1 169 747 B1 proposes to improve frequency tuning by designing the inner conductor of the resonator as a hollow cylinder and by providing an axially adjustable tuning element consisting of a dielectric material inside the inner conductor. In contrast, EP 1 596 463 A1 proposes an adjustable tuning element in the inner conductor that is designed as a hollow cylinder made of a ceramic material, which however is coated with a sleeve-like or pot-shaped tuning body made of metal at its face end extending upwards beyond the inner conductor and across an area that dips deeply into the hollow cylindrical inner conductor. In addition, WO 2004/084340 A1 is referenced which represents and describes adjustable dielectric tuning elements in coaxial filters.
According to EP 1 721 359 B1, a coaxial resonator is to comprise a dielectric layer on the inner side of the cover in a recess provided there to increase its dielectric strength while having a small installed volume.
US 2006/0284708 once again proposes a hollow cylindrical inner conductor in a coaxial resonator with a hollow cylindrical ring placed onto its top annular end face that has the same dimensions as the hollow cylindrical inner conductor, wherein the hollow cylindrical ring consists of a ceramic material with a high dielectric constant. This ceramic ring having a high dielectric constant and low dielectric losses is inserted seamlessly between the open end of the inner conductor of the coaxial resonator and the bottom of the cover. In this way, smaller installed volumes can be attained at the same resonance frequency. In addition, the harmonic waves that can spread in the resonators shift towards higher frequencies.
According to U.S. Pat. No. 6,894,587 B2, both the outer conductor and the cylindrical inner conductor consist of a dielectric substrate. A conductive film for forming the inner conductor and for forming the outer conductor is provided on the respective outer layer of the dielectric material. The coaxial resonator is formed in this way. The dielectric material of the outer conductor comprises an axial hole in which the inner conductor applied onto the inner dielectric material is provided, forming a radial gap.
In addition, we reference U.S. Pat. No. 4,268,809, which describes a filter using multiple coaxial resonators. According to this preliminary publication, a dielectric layer is proposed that jointly covers all free face ends of the inner conductors. Opposite to the inner conductors, a conductive structure is formed on this dielectric layer that is mechanically and galvanically connected to the inner conductor using electrically conductive screws that penetrate the dielectric layer. The conductive structures formed on the dielectric layer end at a spacing from one another, which causes capacitive coupling.
Although smaller filter dimensions are frequently desired, they are either not feasible at all or difficult to achieve. In addition to the maximum permissible insertion loss, one of the factors limiting smaller footprints of the filter assemblies is their maximum rating. The rating of coaxial filters is typically determined by the distance from the free end of the inner conductor to the typically grounded cover and/or the side walls, the tuning elements, etc. A greater distance results in higher potential ratings. Specific minimum distances must be kept depending on the required minimum ratings to prevent destructive microwave breakdowns inside the filter. It is therefore not possible to reduce the size of the filter assemblies any further.
In contrast, it is the object of this invention to provide a generally improved coaxial resonator, particularly for use as a high-frequency filter, that can have a comparatively small installation size even if more complex inner conductor types are used.
As a result of the complete or partial enclosure or coating of the free ends of the inner conductor with a dielectric material whose dielectric constant is greater than 1.2, particularly greater than 2, proposed by the invention, the minimum distances between the cover, the walls and the tuning elements can be reduced even with more complex inner conductor types, since the rating is considerably increased. The enclosure can be achieved using one or more mounted molded parts. It has also proven favorable to extrusion-coat the inner conductor or the essential parts thereof fully or partially with a respective plastic material that has the desired or suitable dielectric values.
The maximum rating can be controlled via the thickness of the dielectric layer. The thicker the layer, the higher the potential ratings. Thinner layers mean smaller dielectric losses and therefore a lower insertion loss for the filter.
In principle, the maximum rating can be influenced by the selection of the dielectric material and its specific properties.
One of the major advantages of the invention therefore is that the volume of the resonator chamber, that is, the installation size of the filter assemblies, can be reduced, resulting in lower overall construction costs. At the same time, the invention permits a higher rating of the filters in a generally simple manufacturing process. Particularly the mounted or extrusion-coated inner conductors form an independent part. The full-area or partial coating or full-area or partial encasing with a respective dielectric material, at least in the area of the free end of the inner conductor, can be provided for any conceivable types of inner conductors.
It is also favorable that the inner conductors used for the resonators of the invention may consist of metal as well as of a dielectric material such as ceramic. One or several or all inner conductors of a respective high-frequency filter can be extrusion-coated. Both originally molded-on inner conductors as well as insertable inner conductors, which can be turned, screwed, pressed into the resonator bottom or otherwise mechanically fastened and galvanically connected, can be encased by casting or pouring. This also results in simple handling since the inner conductor extrusion-coated with the respective sheathing material forms an independent component.
As mentioned above, molded plastic parts can be produced separately rather than provided as molded-on layers and then mounted onto the inner conductor. Molded parts can be provided with respective holders and locking mechanisms which are designed in the shape of fingers and resting, for example, predominantly in radial direction on the inner wall of the housing or the walls and/or are attached with one or several finger-like spacers on the inner or bottom side of the cover.
The advantages according to the invention, that is, a reduction of the installation size, an increase in rating and an improvement of the dielectric strength of each of the resonators can be implemented by the following features of the invention, either alone or particularly in combination: the free ends of the inner conductors of the coaxial resonators are enclosed in a dielectric material εr greater than 1.2, particularly greater than 1.5 or greater than 2, wherein said enclosure of the ends of the inner conductors may be complete or just partial in selected areas;                the ends of the inner conductors with the dielectric material can be enclosed by extrusion-coating or spraying, casting, or painting with suitable plastic materials and/or by mounting special molded parts made of plastic (e.g. using clips);        the insertable inner conductors can be formed in one or multiple parts;        the molded plastic parts can be fastened to the inner conductor or be held on the cover or side walls using molded-on supports or by the specific design of the inner conductor with undercuts into which the molded plastic parts engage;        the inner conductors or ends of inner conductors can be enclosed if the inner conductors are insertable or integrated in. or molded to, the housing (e.g. by casting or pouring);        the insertable inner conductors may consist of metal or a dielectric material (e.g. ceramics);        enclosing can be performed on one, several, or all inner conductors of a respective filter; and        all shapes of inner conductors can be enclosed, there are no limitations in that respect.        